Parathyroid hormone-related peptide (PTHrP) has been hypothesized to play a role in calcium regulation and bone mobilization during lactation. However, evidence to date has been largely circumstantial and insufficient to detect a temporal cause and effect relationship. This study employs a "natural challenge" (i.e., suckling) to elicit the hypothesized biological responses, which will allow us to make inferences about PTHrP, calcium and phosphorus regulation under physiological conditions. The objectives of this study are to determine whether there are increases in serum PTHrP concentrations and indicators of PTHrP activity at the kidney in response to suckling.